Power transformers typically include insulation positioned between or around the various conductive paths therein. The insulation is often formed from a paper or cellulose material. Mineral oil is also commonly used as an insulator in power transformers.
Defects or other abnormalities in the insulation of a power transformer can substantially degrade the performance of the power transformer and, in extreme cases, can cause potentially dangerous malfunctions. For example, moisture and contaminates can degrade the insulating properties of oil, paper, and cellulose insulators. The properties of an insulator can also degrade if the insulator is exposed to voltages greater than its rated voltage. Moreover, defects such as tears, voids, and other flaws, as well as excessive moisture and carbon tracking, can degrade the properties of paper and cellulose insulation. The insulating properties of oil can degrade due to advanced age, overheating, or chemical contamination.
Diagnostic checks are often performed on the insulation of power transformers because of the importance of the insulation to the overall performance of the power transformer. For example, the effectiveness of the insulation in new and newly-refurbished power transformers is usually evaluated before the power transformer is placed in service. Moreover, the insulation of power transformers in the field is often checked on a periodic basis to ensure that the insulation is functioning in a satisfactory manner.
Most commonly-available diagnostic checks for the insulation of power-transformers are adapted to identify the presence of an abnormality, such as a defect, in the insulation system. For example, the so-called “power factor test,” defined by the American National Standards Institute (and described in detail below), is a commonly used diagnostic check. (The power factor test is substantially the same as the so-called “tan-δ” test.) Although diagnostic checks such as the power factor test can indicate the presence of an abnormality, these types of checks often cannot provide an indication of the type of abnormality that may be present.
Identifying a specific type of abnormality often necessitates draining and at least partially disassembling the power transformer. For example, excessive resistance in the core-to-ground circuit of power transformers is sometimes misdiagnosed as excessive moisture in the cellulose insulation of the power transformer. In other words, currently-available diagnostic tests can identify the presence of these types of abnormalities, but often cannot differentiate between the abnormalities. Hence, power transformers are sometimes subjected to a time consuming and expensive drying process based on the mistaken belief that excessive moisture is present in the insulation of the power transformer.
Consequently, a need exists for a diagnostic process that can identify the presence of an abnormality, as well as the type of abnormality, in the insulation system of a power transformer.